loading it by hand via /bin/loadkeys sunt5-uk doesn't generate any command line errors, but it looks like all keys are screwed after that (for example the number keys are producing lower case letters. STOP-A doesn't work at all, that sort of thing)

When I tried to look into this I modded line 37 of /etc/init.d/keymaps so I could control exactly what driver filename was being passed to /bin/loadkeys on startup, rather than messing around with env vars. When it's "/bin/loadkeys sunkeymap" (sans quotes) everything loads up fine (loads of green OKs before I get the login prompt). When it is changed to "/bin/loadkeys sunt5-uk" nearly everything from there onwards in the startup script fails. The keyboard I have is actually a Sun type 6 UK keyboard, but I can only find t5 drivers. Hopefully they are backwards compatible.

What specific log file would I be looking for errors on startup ??

Apologies, I've only just started with Gentoo. I really hope I haven't done something daft!